1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image in an image-forming method such as electrophotography or electrostatic printing, or a toner for forming a toner image in a full-color image-forming method based on a toner-jet system, and particularly, to a toner to be used in a fixing system in which a toner image is fixed to a transfer material such as a print sheet under heat and pressure. The present invention relates also to an image-forming method based on a full-color electrophotographic system to be employed in, for example, a copying machine, a printer, a facsimile, or a digital proof.
2. Description of the Related Art
Various methods have been conventionally known as electrophotographic methods. A general electrophotographic method is as described below. The surface of a latent image bearing member composed of a photoconductive material is uniformly charged by, for example, corona charging or direct charging with a charging roller or the like, and then an electrical latent image is formed on the latent image bearing member by, for example, the application of light energy. Next, the electrical latent image is developed with positively or negatively charged toner so that a toner image is formed. After the toner image has been transferred onto a transfer material such as paper as required, the toner image is fixed onto the transfer material with heat, a pressure, or the like, whereby a copied article is obtained.
In recent years, the formation of an image having an additionally high resolution has been demanded of an image-forming apparatus based on an electrophotographic method such as a printer or a copying machine. In particular, an electrophotographic color image-forming apparatus has been finding use in miscellaneous applications as the apparatus has become widespread, and the demands made upon the apparatus for image quality have become more severe. That is, the color image-forming apparatus has been required to reproduce even a fine portion extremely finely and faithfully in the print of an image such as a general photograph, catalogue, or map. In addition, the apparatus has been required to improve the definition of the color of an image and to extend the color reproduction range of the image.
Further, as for image quality, there are demands for forming an additionally smooth image on a transfer material such as paper even when the transfer material has surface unevenness. In general, an image formed by an electrophotographic method has a step difference between a non-image portion and an image portion in the direction perpendicular to a paper surface of 10 to 30 μm. In a full-color image, in addition to a step difference between a non-image portion and an image portion, a step difference in the image portion between a primary color and a secondary color in the direction perpendicular to a paper surface is 5 to 20 μm, which also causes a reduction in image quality.
In addition, the number of sheets to be printed has also been increasing in association with an increase in speed of an image-forming apparatus, so an additional reduction in running cost has been demanded of the apparatus. Performance requested of toner is as follows: the toner achieves an image with quality and definition each of which is comparable to or higher than a conventional one without narrowing a color reproduction range, a toner consumption is reduced, and fixing energy is reduced.
To satisfy those demands, an increase in content of a colorant in toner has been proposed (see, for example, Patent Documents 1 to 4). Each of those documents aims to form an image with a smaller toner amount than a conventional one and to reduce the surface unevenness of the image by increasing the content of a colorant in toner. However, an increase in colorant content of toner has involved the following problem: the peak of a characteristic absorption wavelength resulting from a colorant in the reflection spectrum of an image becomes broad, with the result that the chroma and lightness of the image reduce.
There is employed a technology involving controlling the dispersed state of a colorant in toner as a method of suppressing reductions in chroma and lightness of a toner image (see, for example, Patent Document 5). The control of the dispersed state of the colorant in the toner exerts a certain effect in some cases, but the control is still insufficient for forming of an image with small image unevenness while reducing the usage of the toner, and, in the case of the control, a reduction in chroma of a secondary color is particularly remarkable.
As described above, no toner having the following characteristics has been found yet: an image having a high resolution and high definition is achieved, good image quality is expressed while none of an image color gamut, chroma, and lightness is reduced even in a secondary color, and a running cost can be reduced.
Patent Document 1: 11-72960 A
Patent Document 2: 11-237761 A
Patent Document 3: 2002-131973 A
Patent Document 4: 2005-128537 A
Patent Document 5: 2003-280723 A